Arrangement for detecting slippage of wheels of vehicles

ABSTRACT

A slippage detecting system for use in conjunction with motor vehicles. Each wheel of the vehicle is provided with a transducer which supplies a voltage dependent upon the speed of the wheel. The transducer output is applied to a Schmitt-trigger circuit which becomes actuated when the voltage exceeds a predetermined value. Upon actuation of the Schmitt-trigger circuit, the braking system of the motor vehicle is operated through an amplifier, so that the braking force is regulated to prevent locking of the wheels. The transducer is in the form of a voltage generator with regulated excitation voltage. The voltage regulator includes an integrator and is connected between the output of the generator and the excitation winding thereof.

United States Patent [151 3,659,907 Gunsser et al. 51 May 2, 1972 [54]ARRANGEMENT FOR DETECTING 2 5 32 g s! RIM 3,0 6,148 3 l 2 uo.........303/2l CF I 'SLIPPAGE 0F WHEELS O VEHICLES 3,494,671 2/1970Slavln etal. .......,......,......,..303/2l P [72] Inventors: PeterGunner; Rlchard'Zechnnll, both of 3,511,542 5/1970 Fielek, ..303/2l CGStuttgart Primary Examiner-Milton Buchler [73] Assignees: Klauschrlagbsetuttigart-hDrrlglvzargd, all of, Asst-am Examiner stephen GKunin Germany o rt m umgan' Attorney-Michael S. Striker Germany [22]Filed: Dec. 4, 1969 [57] ABSTRACT 2 APPL M 332,101 A slippage detectingsystem for use in conjunction with motor vehicles. Each wheel of thevehicle is provided with a transducer which supplies a voltage dependentupon the speed of Fwd!" Apluc'mm Pmmy the wheel. The transducer outputis applied to a Schmitt- 9 1968 Germany g 13 5 5 trigger circuit whichbecomes actuated when the voltage exv ceeds a predetermined value. Uponactuation of the Schmitt- [52] U.S.Cl. ..303/21 CF, 180/105 E, 303/20,trigger circuit, the braking system of the motor vehicle is 317/5,318/52, 324/161, 340/263 perated through an amplifier, so that thebraking force is 51 int. Cl. ..Bt s/os regulated to Prevent locking ofthe whcels- The transducer is [53] Field 0{Search 138/181; 303/21, 324/A, in the form of a voltage generator with regulated excitation 324/70 G160, 317/5; 0 2 0 voltage. The voltage regulator includes an integratorand is 340/52 R, 313/52 connected between the output of the generatorand the excitation winding thereof. [56] References Cited 7 Claims, 6Drawing Figures UNITED STATES PATENTS 3,563,611 2/1971 Sharp ..303/21 CF54 came/woe FEOCEJJ/IVG $72465 //YTG'A 70R 70 AMPL lF/EQ Patented May 2,1972 2 Sheets-Sheet 2 Haw (WP/57' ARRANGEMENT FOR DETECTINGSLIPPAGE OFWHEELS OF VEHICLES BACKGROUND OF THE INVENTION the driving path. Thecoefficient'of friction between a sliding wheel and the driving path isconsiderably lower than the coefficient of friction between a rollingwheeland the driving path. The wheels must also be prevented fromlocking. from the viewpoint of driving safet since the vehicle caneasily skid during such condition. A wheel always becomes locked whenthe force directed along the drive path and applied to the wheel throughthe pressure between the brake shoes and the brake drum, becomes largerthan the component directed perpendicular to the wheel axle arising fromthe coefiicient of friction between the wheel and'the driving path. Thiscomponent is, furthermore, proportional to the loading of the wheel forrealizing the maximum possible static frictional force.

When braking a vehicle on a straight path, the rear wheels become freedfrom load whereas the front wheels have load applied to them, during thebraking'or deceleration of the vehicle. In order to avoid the conditionthat the rear wheels become locked, the braking force applied to thesewheels must be comparably reduced to the braking force applied to thefront wheels. For purposes of increasing the safety conditions whenturning or going around a curve while braking, the

individual wheels must be difierently braked while such br ing action isin progress. In particular, the wheels prevailing at the outer side ofthe curve must be braked more severely than the wheels lying along theinner side of the curve.

The problems prevailing during the braking of a motor vehicle, alsoappear when the vehicle is accelerated. Thus, when accelerating thedriving wheels of motor vehicles, it is possible that the latter mayskid. At the same time, considerable time losses may be incurred withrail cars during the accelerating phase when the driving wheels arerotated. Braking regulating systems for preventing and regulatingslippages are known in the art. In these conventional systems, thebraking of the wheels is made automatically dependent upon thedeceleration of the wheels when excessive. Such a deceleration of therotational speed of the wheels indicates the beginning of a slippagephase. Electrical transducers are also known in the art which include acontact for providing which becomes actuated through the relative motionbetween a swinging mass and the driving shaft of the wheels, when theacceleration is excessive.-

All of these conventional arrangements known in the art,

have the disadvantage that the transducers which provide signals forexcessive deceleration or acceleration do not operate so that they arefree from wear. Furthermore, these arrangements have the disadvantagethat they are not provided with a reliable time constant to the extentthat they will function without delay at the beginning of a slidingmotion when braking or accelerating.

Accordingly, it is an object of the present invention to provide anarrangement which detects the danger of slippage of the wheels of motorvehicles, so that operation of the detection system takes place withoutwear and without substantial delay.

the generator is made dependent upon the surface speed or velocity ofthe wheels. The signal processing stageconnected electrical pulses andjects and advantages thereof, will to the generator output is providedwith logical switching elements, and'becomes influenced throughvariations in the out- Schinitt-trigger circuit is influenced by thevoltage thereof. A

voltage regulator with integral control, furthermore, is associated withthe generator, and the input of the signal processing stage is directlyconnected to the output of the generator.

SUMMARY OF THE INVENTION An arrangement for preventing skidding orslippage of motor vehicles. Each wheel of the vehicle is provided, witha transducer in the form of a voltage generator which transmits avoltage dependent upon the speed of the wheel. The output of the voltagegenerator is applied to a Schmitt-trigger circuit which in turn, emits asignal when the applied voltage exceeds a predetermined value. Throughthe application of an amplifier, the Schmitt-trigg er circuit operates abraking system which in turn applies a braking force to the braking drumof the vehicle, so that locking of the wheels is prevented, and therebypossible skidding and sliding of the vehicle are also prevented. Avoltage regulator connected to the output of the generator alsoregulates the voltage thereof in conjunction with an integral controlcircuit. At the same time, the generator output is connected directly tothe Schmitt-trigger circuit. The voltage'regulator is used to regulatethe excitation current for the generator. v

The novel features which' are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional obbe best understood from thefollowing description of specific embodiments when read in connectionwith the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block of the arrangementfor detecting slippage of a motor vehicle, in accordance with thepresent invention; I v J a FIG. 2 is a block diagram of the circuitdetails for a voltage generator used in conjunction with the arrangementof FIG. 1;

FIG. 3 is an electrical circuit diagram and shows the integrating stageused in conjunction with the regulation of the voltage of the generatorin FIG. 2;

FIG. 4 is a functional diagram and shows the construction of the voltagegenerator used in conjunction with the arrangement of FIG. 1;

FIG. 5 is a graphical representation of voltage signals prevailing inthe circuitry of FIG. 2; and

FIG. 6 is a schematic diagram of an arrangement using the voltagegenerator of FIG. 4 when operating in conjunction with the arrangementof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS brake drama, by way of linkage16 on which the brake shoes 17 are arranged. The vehicle axle 1 3 iscoupled to the mechanical input of a generator 18, whereas a processingunit or evaluating circuit 19 is conn'ecte d to the electrical output ofthe generator l8. The output'of the processing 'or evaluating circuit 19is connected to a first input of the brake cylinder arrangement 15. Thesecond input from this brake cylinder arnormal I rangement 15, isderived from the output of a braking force transducer 21. This brakingforce transducer 21 includes, for example, a hydraulic pressurecylinder, in which case theinterconnecting line between the transducer21 and the braking cylinder arrangement is preferably constructed in theform of a tube or hose for conducting the fluid under pressure. Thebraking force transducer 21, furthermore, is linked to the brake pedal23 through linkage 22. The brake pedal 23 is movably arranged orpivotally mounted on the vehicle chassis 24.

When the driver wishes to brake the motor vehicle equipped with thebraking arrangement shown in FIG. 1, he depresses the brake pedal 23.The braking force applied to the pedal, becomes transmitted to thevehicle wheels, through the linkage 22 and the braking force transducer21. In particular, a braking force is applied to the braking disc 14,through the brake cylinder arrangement 15, the linkage 16 and the brakeshoes 17. With the braking force applied to the disc 14 in this manner,the axle 13 of the wheel 11 is decelerated. If the braking action is toosevere, the motor vehicle wheel 11 may become entirely blocked orstopped, so that it no longer rolls upon the surface or path 12, butinstead slides therealong. Such sliding of the wheel has theaccompanying disadvantages enumerated above.

The generator 18 coupled to the vehicle axle 13 produces an outputvoltage, the unregulated value of which is dependent upon the surfacevelocity of the wheel 11. Through means of an associated voltageregulator, this output voltage from the generator is maintainedsubstantially constant during accelerating and decelerating intervals.The processing circuit 19 is made dependent upon the magnitude of theoutput voltage from the generator 18. When this output voltage from thegenerator drops below a predetermined level, the unit 19 transmits tothe brake cylinder arrangement 15, a signal which results in a reductionof the braking force applied by the shoes 17 to the disc 14. When thevehicle speed decreases and the surface velocity or speed of the wheel11 thereby decreases correspondingly, the output voltage from thegenerator also decreases. For this reason, however, the voltageregulator is provided which maintains the generator voltage outputsubstantially constant for relatively small variations in the surfacevelocity of the vehicle wheel 1 1.

In the block diagram of FIG. 2, the output of the generator 18 is shownto be connected with the input of the processing stage 19 which containsa Schmitt-trigger circuit. The output of the signal generator 18 is,furthermore, connected to the input of a voltage regulator 28 whichcontains a regulating stage 29. The latter, in turn, influences theexcitation current of the generator 18. The voltage regulator 28,furthennore, includes an integrating stage 32 which is connected to theinput of the regulating stage 29. The output of the processing stage 19,furthermore, is connected to the input of an amplifier 33.

FIG. 3 shows the details of the integrating stage 32. A first resistor37 is connected between the input tenninal 35 of the integrating stage,and a circuit junction 36. A first capacitor 39, furthermore, isconnected between this circuit junction 36 and a common return line 38.A second resistor 43 is connected between the output terminal 42 of theintegrating stage, and this same circuit junction 36. A second capacitor44 is arranged between this output terminal 42 and the common returnline 38. FIG. 3 illustrates only a simple embodiment in which theintegrating stage may be designed or constructed. Other well-knownarrangements may, of course, also be applied for this'use or purpose,and this circuit may, for example, be in the form of a semi-conductorMiller integrator.

FIG. 4 shows an embodiment of an arrangement for regulating the brakingforce applied, in accordance with the present invention, through the useof a generator. A transducer wheel 46, for example, is mechanicallycoupled to the axle 13 of the motor vehicle, and is made offerromagnetic material. This transducer wheel 46 carries projections inthe form of teeth 47, for example, along its periphery. A core or yoke48 extends across the transducer wheel and is horseshoe shaped orU-shaped. The'poles of the core lie across either a tooth 47 of thewheel 46, or a space between teeth. A constant D.C. current flowsthrough the coil 49. Depending upon the position of the transducer wheel46, the inductance of the coil 49 varies. Thus, the inductance will varydepending upon whether the magnetic circuit is closed through the wheel46, the core 48, and the teeth 47, or whether this magnetic circuit isopened by being directed through the gaps or spaces between the teeth.

If the transducer wheel 46 rotates with respect to the core or yoke 48,the inductance of the coil 49 oscillates periodically, and analternating current becomes superimposed onto the direct current flowingthrough the coil 49. The alternating voltage components resulting fromthe current oscillations become capacitatively filtered out across theterminals 51 and 52 of the coil 49. A different design is also possible.Thus, the transducer wheel 46 carries a number of magnets 47 about itsperiphery, and these magnets are arranged with alternating polarity. Themagnet 47 can be mounted individually upon the wheel through amechanically adapted material. At the same time, the wheel 47 mayconsist, on the other hand, of permanent magnetic material, and themagnets at the periphery .of the wheel where magnetically applied in thefabrication of the wheel. A core 48 extends, again, across the wheel 47,so that one pole of the core lies opposite a magnet of one polarity, andthe second pole of the core lies across a magnet of opposite polarity.An air gap prevails between the poles of the core 48 and the magnets. Acoil 49 is arranged upon the core 48. When, now, the transducer wheel 46rotates relative to the core 48, positive and negative pulses arerealized at the output terminal 51 and 52 of the coil 49. With increasein the rotational speed of the transducer wheel 46, the frequency ofthese pulses increases correspondingly.

The arrangement in accordance with the present invention, will now bedescribed in conjunction with the block diagram of FIG. 2. The generator18 which may be designed, for example, in the form of a D.C. generatorwith separate excitation, generates a voltage which is proportional tothe rotational speed of the generator shaft when rotating in a constantmagnetic field due to the excitation winding. The output voltage fromthe generator is applied to the input of voltage regulator 28 whichcompares this generated voltage with a predetermined desired voltagevalue. As soon as the output voltage of the generator 18 rises, theregulating stage 29 reduces the current through the excitation windingof the generator 18, and as a result the output voltage from thisgenerator again decreases. In order to avoid oscillations, anintegrating stage 32 is connected in front of the regulating stage 29.When the rotational speed varies rapidly, the output voltage of thegenerator 18 cannot be immediately regulating or contemplated, since themagnetic field of the excitation coil cannot vary in a stepwise manner.The regulating stage 29 is, furthermore, associated with a predeterminedtime constant. Consequently, in view of the integrating stage 32connected in front of the regulating stage 29, a deviation of differenceexists between the generator output voltage and the desired voltageduring rapid variations of the rotational speed.

The input of the processing stage 19 is directly connected with theoutput of the generator 18. As already indicated, the stage 19 isequipped with a Schmitt-trigger circuit. When the rotational speedvariations of the vehicle wheel 11 lie within permissible limitsestablished for normal braking or normal acceleration, theSchmitt-trigger circuit within the unit 19 assumes a stable state inwhich it transmits no signal to the amplifier 33. If the rotationalspeed of the vehicle wheel 11 changes from a previous quantity as in thecase of the transition from static friction to kinetic friction, thevoltage regulator 28 does not tend to compensate the output voltage fromthe generator 18 with sufficient rapidity. Accordingly, the outputvoltage drops below the predetermined level at the Schmitt-triggercircuit within the unit 19, and as a result the latter is actuated andtransmits a pulse. This process is shown in FIG. 5, in which the curve118 shows the output voltage of the generator 18, and the curve 119represents the output voltage of the Schmitt-triggercircuit 19. Both ofthese curves are illustrated as a function of time t. Direct actuationof the braking system is then realized through the amplifier 33 and afollowing electromechanical converting unit.

The circuit arrangement of FIG. 2 is designed so that small variationsin the output voltage 118 are of no consequence since they do notproduce actuation or switching of the Schmitt-trigger circuit within theunit 19. Accordingly, a measure of safety is realized from the viewpointof fabricating tolerances. In the schematic circuit diagram of FIG. 6,one end or terminal of the excitation winding 49 of the generator 18, isconnected to the positive terminal of a current source, not shown. Theother end or terminal of the excitation winding 49, on the other hand,is connected to the collector of a transistor 55. A stabilizing resistor56 is connected between the emitter of the transistor SS and thenegative terminal of that current source, not shown. A capacitor 57 isconnected, furthermore, to the collector of the transistor 55 and theinput of a rectifier 58. The output of this rectifier is connected toboth the input of the Schmitt-trigger circuit within the unit 19, andthe input of the integrator 32. The output of this integrating circuit32 is connected to the base of a transistor 55.

When the vehicle is stationary, current flows from the positive terminal54 to the negative terminal 59, through the path of the excitationwinding 49, the transistor 55, and the resistor 56. When the transducerwheel 46 rotates with substantially constant speed, the inductance ofthe excitation winding 49 varies or oscillates synchronously. As aresult, the current flowing through the excitation winding 49 is alsosubjected to oscillations. The alternating voltage components whicharise therefrom, can be filtered out at the terminals of the excitationwinding 49. The alternating voltage components are applied to therectifier 58, through the capacitor 57, and the rectifier thereby emitsa pulsating D.C. voltage. The integrating stage 32 forms, thereby, theaverage value, and establishes the operating point of the transistor 55,thereby, from this average D.C. voltage. When the transducer wheel 46turns more rapidly, the alternating voltage components across thecapacitor 57 have also'a higher or greater magnitude. This leads to ahigher pulsating D.C..voltage at the negative output of the rectifierarrangement 58, and thereby to a more negative value or level at theoutput of the integrating stage 32. The operating point of thetransistor 55 is thereby shifted so that a lower current flows throughthe excitation winding 49. As a result, the alternating voltagecomponents appearing across the capacitor 57 are also made smaller. Whenthe transducer wheel 46 rotates more slowly, the alternating voltagecomponents drop in magnitude, and the pulsating D.C. voltage becomesthereby smaller. The operating point of the transistor 55 is finallydisplaced or shifted so that the current through the excitation winding49 increases which, in turn, increases the alternating voltagecomponents again.

As long as the transducer wheel 46 is decelerated by a substantiallysmall amount, the entire regulating loop through the rectifier 58,integrating stage 32 and transistor 55, has sufficient time to maintainthe variations of the alternating voltage components across thecapacitor 57 sufficiently small. When the deceleration of the transducerwheel 46, however, exceeds a predetermined value, a sharp step variationappears in the D.C. voltage at the output of the rectifier 58. Thisresulting D.C. step voltage remains until the operating point of thetransistor 55 is again shifted through the integrating stage 32. Withsuch a severe voltage change, however, the voltage drops below thethreshold level of the Schmitt-trigger circuit within the stage 19, andas a result the Schmitt-trigger circuit transmits a signal to theamplifier 33. This output signal then operates upon the brake system 15of the motor vehicle, before the wheel 11 has become completely locked.Prior to the instant that the motor vehicle becomes absolutelystationary, the arrangement is rendered inoperative.

The arrangement in accordance with the present invention, has a numberof further advantages. Thus, a transducer is used which is free fromwear and which will be operational for the operating life of thearrangement, and will be thereby operating reliably during this period.The arrangement of the present invention, operates also practicallywithout any delay times so that the system function in the braking ordriving mode when the vehicle first commences to slide. The digitalregulating process, furthermore, has the advantage that the operation iscarried out without being .subjectable to the influences of noiseeffects. The response to stray pulses, and oscillations appearing acrossthe voltage supply line when using conventional accelerometers withconventional differentiating stages, is avoided through the presentinvention. The arrangement of the present invention also takes intoaccount the conditions that an increase braking force is required tohold or lock the wheels when the weight of the vehicle increases throughincreased load, and the vehicle is braked. At the same time, the presentarrangement also takes into account the condition that an increasedoperating or driving force is required to rotate the wheels whenaccelerating such a heavier vehicle. The same arrangement provides,therefore, for reducing the danger resulting from locking the wheels,and also reducing the danger resulting from turning the drive wheels ofthe ,vehicle during acceleration. The present invention, furthermore,will operate independently of whether the driving path has a dry surfaceor is covered with ice.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been'illustrated and described as embodied inslip detection arrangement for motor vehicles, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can be applying.

current knowledge readily .adapt it for various applications withoutomitting features that, from the standpoint of prior art, fairlyconstitute essential characteristics of the generic or specific aspectsof this invention and, therefore, such adaptations should and areintended to be comprehended within'the I meaning and range ofequivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. An arrangement for detecting the slipping or locking of at least onewheel on a surface, comprising, in combination, alternating currentvoltage generating means mechanically coupled to said wheel forproviding an alternating current voltage dependent upon the speed ofsaid wheel; rectifying means connected to the output of said alternatingcurrent voltage generating means for rectifying the alternating currentvoltage of said generating means to provide at its output a pulsatingdirect current voltage; integrating means having an input connected tothe output of said alternating current voltage generating means and tothe output of said rectifying means; voltage regulating means having aninput connected to the output of said integrating means and an outputconnected to the input of said rectifying means for maintainingsubstantially constant, the voltage output of said alternating currentvoltage generating means except for sudden changes in the speed of saidwheel associated with slipping or locking of the same; and signalprocessing means connected to the output of said rectifying means, saidsignal processing means including a Sehmitt trigger connected ,toreceive as input the output of said rectifying means for actuation so asto provide a signal whenever said voltage output of saidalternating'current voltage generating means suddenlychanges by anamount sufficient to indicate slipping or locking of said wheel.

2. The arrangement as defined in claim 1 wherein said voltage generatingmeans includes a rotatable member with projections of ferromagneticmaterial and mechanically coupled to said wheel; and magnetic circuitmeans mounted on the axle of said wheel and in the near vicinity of saidprojections on said rotatable member.

6. The arrangement as defined in claim 1 including braking means coupledto said wheel for braking the rotation thereof, the braking forceapplied to said wheel being dependent upon a signal provided by saidsignal processing means.

7. The arrangement as defined in claim 1, wherein said alternatingcurrent voltage generating means includes a coil having induced thereinsaid alternating current voltage when said wheel rotates, and whereinsaid voltage regulating means includes a transistor having an input andan output, said input being connected to the output of said integratingmeans so that the conductivity of said transistor varies in dependenceon the output of said integrating means, said output of said transistorbeing connected to said rectifying means, said coil being connected inseries with the emitter-collector path of said transistor so that thedirect current flowing through said coil varies in dependence on theconductivity of said transistor, whereby the value of said alternatingcurrent voltage is maintained substantially constant for all changes ofwheel speed except for sudden changes associated with slipping orlocking, which sudden changes cause actuation of said Schmitt trigger.

i i i i

1. An arrangement for detecting the slipping or locking of at least onewheel on a surface, comprising, in combination, alternating currentvoltage generating means mechanically coupled to said wheel forproviding an alternating current voltage dependent upon the speed ofsaid wheel; rectifying means connected to the output of said alternatingcurrent voltage generating means for rectifying the alternating currentvoltage of said generating means to provide at its output a pulsatingdirect current voltage; integrating means having an input connected tothe output of said alternating current voltage generating means and tothe output of said rectifying means; voltage regulating means having aninput connected to the output of said integrating means and an outputconnected to the input of said rectifying means for maintainingsubstantially constant the voltage output of said alternating currentvoltage generating means except for sudden changes in the speed of saidwheel associated with slipping or locking of the same; and signalprocessing means connected to the output of said rectifying means, saidsignal processing means including a Schmitt trigger connected to receiveas input the output of said rectifying means for actuation so as toprovide a signal whenever said voltage output of said alternatingcurrent voltage generating means suddenly changes by an amountsufficient to indicate slipping or locking of said wheel.
 2. Thearrangement as defined in claim 1 wherein said voltage generating meansincludes a rotatable member with projections of ferromagnetic materialand mechanically coupled to said wheel; and magnetic circuit meansmounted on the axle of said wheel and in the near vicinity of saidprojections on said rotatable member.
 3. The arrangement as defined inclaim 1 wherein said signal processing means comprises logical switchingcomponents at least partially of semi-conductor construction.
 4. Thearrangement as defined in claim 1 including a motor vehicle driven bysaid wheel and braked by a signal provided by said signal processingmeans.
 5. The arrangement as defined in claim 1 including rail-operatedvehicle means driven by said wheel, the accelerating forces for saidvehicle being regulated by a signal provided by said signal processingmeans.
 6. The arrangement as defined in claim 1 including braking meanscoupled to said wheel for braking the rotation thereof, the brakingforce applied to said wheel being dependent upon a signal provided bysaid signal processing means.
 7. The arrangement as defined in claim 1,wherein said alternating current voltage generating means includes acoil having induced therein said alternating currEnt voltage when saidwheel rotates, and wherein said voltage regulating means includes atransistor having an input and an output, said input being connected tothe output of said integrating means so that the conductivity of saidtransistor varies in dependence on the output of said integrating means,said output of said transistor being connected to said rectifying means,said coil being connected in series with the emitter-collector path ofsaid transistor so that the direct current flowing through said coilvaries in dependence on the conductivity of said transistor, whereby thevalue of said alternating current voltage is maintained substantiallyconstant for all changes of wheel speed except for sudden changesassociated with slipping or locking, which sudden changes causeactuation of said Schmitt trigger.